fix(docs): add missing line in the fiptool command for stm32mp1

Add the missing trusted key certificate in the fiptool
command line.

Signed-off-by: Lionel Debieve <lionel.debieve@foss.st.com>
Change-Id: Ife95b0261f04b7fd07a9b01488f9e5be9b87e841
diff --git a/docs/plat/st/stm32mp1.rst b/docs/plat/st/stm32mp1.rst
index 35e8f8c..b6e4b0d 100644
--- a/docs/plat/st/stm32mp1.rst
+++ b/docs/plat/st/stm32mp1.rst
@@ -205,6 +205,7 @@
         --nt-fw <u-boot_directory>/u-boot-nodtb.bin \
         --hw-config <u-boot_directory>/u-boot.dtb \
         --fw-config build/stm32mp1/release/fdts/fw-config.dtb \
+        --trusted-key-cert build/stm32mp1/release/trusted_key.crt \
         --tos-fw-cert build/stm32mp1/release/tos_fw_content.crt \
         --tos-fw-key-cert build/stm32mp1/release/tos_fw_key.crt \
         --nt-fw-cert build/stm32mp1/release/nt_fw_content.crt \